Single-phase alternating current electric motors conventionally are provided with two windings on a stator core, inductively coupled to the rotor of the motor. Such motors are widely used for a variety of different purposes and range in size from very small fractional horse-power motors on up to multiple horse-power sizes. Single-phase motors are particularly popular since most home and business alternating current supplies are in the form of single-phase power supplies.
Electric motors typically include a stator, which is wound with start windings and run windings connected to the source of operating power. The stator windings surround a rotor, which rotates a shaft to produce the motor output. Rotors are made in a number of different configurations, such as squirrel cage rotors, high resistance rotors, low resistance rotors, wound rotors or multiple winding high and low resistance rotors. All of these configurations, along with various stator winding arrangements, are well known in the electric motor industry. Typically the start winding is made of relatively small diameter wire and the run winding is made of relatively large diameter wire, compared to the diameter of the start winding. These windings are angularly displaced from one another on the stator.
The above identified co-pending application is directed to a modified electric motor which does not employ switches in the starting circuit, but which instead utilizes two series-connected windings (of substantially the same diameter heavy wire) electrically phase displaced 90.degree. from one another on the stator core. One of these windings has a capacitor connected in parallel with it to form a parallel resonant circuit at the operating frequency of the motor. The motor of this co-pending application is a high efficiency motor which overcomes most of the disadvantages of prior art capacitor start/capacitor run motors. One disadvantage, however, which is present in the motor in the above-identified co-pending application is that the starting torque of the motor is relatively low. As a consequence, applications for the motor of applicant's co-pending application are primarily for situations which do not require a very high starting torque, such a pumps, blowers, machine tools and many commercial and domestic appliances.
A motor which has the advantages of the parallel resonant motor of the above-identified co-pending application and which further is capable of producing high starting torque to permit use of such a parallel resonant motor in applications where high starting torque is necessary is disclosed in the above-identified U.S. Pat. No. 4,675,565. The motor of this patent employs the same basic resonant circuit disclosed in the above-identified co-pending application. In addition, however, a starting capacitor is connected in series with a switch in parallel with the capacitor forming a portion of the LC resonant circuit for the motor. This switch is closed during start-up of the motor and is opened during normal load conditions of operation of the motor once the motor has attained its operating speed. The motor of U.S. Pat. No. 4,675,565 produces efficiency similar to the motor of the co-pending application, but is capable of use in situations where high starting torque is desired. The motor of U.S. Pat. No. 4,675,565 is similar to the one of the co-pending application inasmuch as no separate start winding is switched into and out of the circuit during the motor operation. Both the start and run windings are continuously connected in series circuit throughout both the starting and run operation sequences of the motor. Only the starting capacitor is switched out of the circuit upon attainment of operating speed of the motor.
For larger horse-power motors, the capacitor discharge of the LC resonant circuit capacitor when the motor is turned off or switched off creates undesirable noise and chattering or vibration of the rotor. This results from the discharge of the capacitor through the winding across which it is connected.
It is desirable to provide a motor which has the advantages of the parallel resonant motors of the above-identified co-pending application and patent, and which further does not have the disadvantages of noise and chattering when the motor is turned off.